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SLEAC investigation

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Ali Ousmane

statisticien

Normal user

2 Feb 2017, 11:33

Hello everyone
I would like to know if it is necessary to calculate the confidence interval for a sleac investigation in one health district.
Thank you

Mark Myatt

Consultant Epideomiologist

Frequent user

2 Feb 2017, 12:49

If you just have a single small sample for the one district (rather than several small samples from one district) then a confidence interval will be very wide. This is pushing the data to do something it was not design to do. With the SLEAC classifier we classify (low, moderate, high) based on our thesholds. This requires a lot less data to do than to create an estimate with a usefully narrow confidence interval.

If (e.g) you have thresholds at 20% and 50% so that:

Low : < 20%
Moderate : 20% - 50%
High : > 50%

and have n = 36 and 12 covered cases you would report a moderate coverage classification. It is simple and quite error free if there are not a very large number of SAM cases in the district.

A 95% CI might be:

  p = 12 / 26 = 33.33%
  SE = sqrt((0.3333 * (1 - 0.3333)) / 36) = 0.0786
  LCL = 0.3333 - 1.96 * 0.0786 = 0.1792
  UCL = 0.3333 + 1.96 * 0.0786 = 0.4874

That's 95% CI = 17.9% - 48.7%. We're no longer very sure that it is moderate.

We can try to fix this by applying a finite population correction. If we assume that there were 130 cases of SAM in the population at the time of the survey (we can get this as population * prevalence) then:

  p = 12 / 26 = 33.33%
  SE = sqrt((0.3333 * (1 - 0.3333)) / 36) * sqrt((130 - 36) / (130 - 1))
  SE = 0.0671  
  LCL = 0.3333 - 1.96 * 0.0671 = 0.2018
  UCL = 0.3333 + 1.96 * 0.0671 = 0.4448

We are now pretty confident of our moderate classification but (looking at the 95% CI) we do not have a great deal more information. The SLEAC classifier says "between 20% and 50%" and the estime with 95% CI tells us "its between about 20% and 45%".

The best thing to do (IMO) is to take the SLEAC classification and forget estimation.

If you have many small sampled (e.g. one per clinic catchment area or one per district) then the samples can be pooled and an overall estimate and 95% CI calculated.

I hope this is of some use.

Lio

CMAM advisor

Technical expert

2 Feb 2017, 13:01

Dear Ali,

The sample size of a SLEAC survey is generally of around 40 children, this is enough to be able to classify coverage within a category (low, medium, high - which is determine before the exercise), no C.I is calculated for a category. However, in the event that during the field work you find more than 96 children (probably by having underestimated the prevalence) it is then possible to calculate an overall coverage estimate, contrary to a "classification", in this case you can and you should calculate the C.I. (Note: the same applies if SLEAC covers more than one district).
All the best

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